Update out-of-date samples

[martincostello]

As reported in #852 (comment), some of the examples in the wiki are out-of-date for Polly v7.

These examples are outdated and the wiki also points to this outdated issue (https://github.com/App-vNext/Polly/wiki/Retry#retryafter-when-the-response-specifies-how-long-to-wait).

I was hoping that that applying WaitAndRetryAsync on the PolicyBuilder would still work in 7.2.2, but the return type is now AsyncRetryPolicy.

A quick excerpt from #414 that doesn't work anymore:

var _retryAfterPolicy = Policy .HandleResult(r => r?.Headers?.RetryAfter != null) .WaitAndRetryAsync( retryCount: numRetries, sleepDurationProvider: (retryCount, response, context) => getServerWaitDuration(response) );

If you can update the wiki to point to the new overload and to a more recent working example, it would be greatly helpful!

We should go through the wiki and update any out-of-date examples found with up-to-date equivalents.

[martincostello]

Superseded by #1091.